Category Archives: Cylinder Unit

How to dress up the scoring marks on the cylinder liner?

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This post is about temporary repairing the cylinder liner in case of scoring which if is detected at an early stage and if appropriate countermeasures are taken, sudden severe wear (scuffing) , losing liner and piston rings, can be avoided and the reliability improved. Visual piston underside inspection is recommended to be carried out at every four to eight weeks.

Even though the surface appears to be in good condition at first glance, the rough surface and sharp vertical scars generated by foreign particles and minor abrasion between liner and piston rings may emerge partially in the cylinder liner. In the worst-case scenario, this will result in excessive wear on the cylinder liner and piston rings (scuffing). It is therefore recommended to inspect the liner surface carefully whenever opening the cylinder cover. If the liner surface is rough and has sharp vertical scars, this must be dressed up by using hand-honing and it is especially recommended when burnt markings and/or deep vertical scars on the surface of piston rings are discovered, since this indicates the similar condition on the cylinder liner surface.

Scuffing marks on piston ring

Scuffing marks on cylinder liner

For such kind of scoring marks, the intention is to break the surface of such scored liner structure and not to remove them completely as this might result in a liner shape that is too uneven and causes blow-by.

The tool necessary for hand-honing comprises of:

  • angle grinder – 100 mm or 180 mm diameter
Air driven angle grinder
  • emery cloth disc – grit size #80 and #120

In order to perform a correct hand-honing you must take into the consideration the following procedure:

  • Hand-honing must be applied to the liner surface in the whole circumference and full length of piston stroke as much as possible.
Dress up by hand-honing
Dress up scoring
  • If the piston is installed, it needs to be cover up to avoid dirt ingress between piston, piston rings and liner.
  • The cylinder liner surface should be obliquely honed (WW shape)
Source: Wartsila – Direction of hand-honing
  • The sharp ridges should be honed half of their height. The honing should be finished when the liner surface become smooth (e.g. no cotton fiber stuck on surface).
Source: Wartsila – Depth of hand honing (section view)
  • In case of extremely hard, blue colour patches all protruding lines have to be grinded. The dressing up should be done by disk cutter, followed by polishing with emery cloth disc. The disc cutter, of 100 – 250 mm diameter and maximum thickness of 2 mm, should be used in the flattest possible angle to obtain a smooth surface.
  • Clean all around the liner after honing by wiping the cylinder liner surface, cylinder oil groove and scavenging ports.

In conclusion, cylinder liner dress up is a temporary and an emergency countermeasure used in order to save the liner. Normally in these cases, cylinder liner need to be removed and re-honed in a specialized workshop or in-situ onboard by utilizing a special honing tool which is not easily available for large bore engines. The job is time consuming and requires adequate skills.

However, before honing the cylinder liner must fulfil few conditions:

  • No cracks are present
  • If the liner wear is eccentric and more than 1 mm out of centre, then honing is not recommended. This is difficult to measure, but one method is to compare the thickness of the wear edges around the liner.
  • From TDC of the top piston ring wear edge to about mid-stroke, the inside diameter should not exceed that for a new liner by the value shown in table below (Source: Wartsila. For MAN engines the values might be slightly different)
  • From about mid-stroke to the bottom of the liner, the inside diameter should not exceed that for a new liner by the value shown in the table below (Source: Wartsila. For MAN engines the values might be slightly different)
Source: Wartsila
  • O-ring grooves must be in good condition

Very important – Cylinder liners which exceed the above mentioned diameter limits should be scrapped!

In conclusion, in order to run the engine efficiently and to avoid engine breakdowns and damage, proper maintenance and periodical checks must be carried out as per your company and manufacturer’s maintenance plan and instructions. Moreover, every engineer, especially senior engineers and juniors who aim for promotion, must be familiar with proper monitoring, inspections and how to interpret the findings as they offer valuable information about engine running condition.

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Wear edge in the cylinder liner and why we need to remove it?

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Wear edge is formed in the top position of the upper piston ring when the cylinder liner is worn after many thousands of hours of service or from abnormal running conditions, such as scuffing or corrosion.

Piston top in TDC

With heavily worn cylinder liners, it is a necessity to remove the edge prior pulling out the piston, otherwise the piston will not pass and in most cases will lift the cylinder liner. If this happens, will then have to pulled the cylinder liner in order to renew the O-rings.

Some of the engine manufacturers, recommends that these wear edges to be removed when replacing the piston rings and a groove to be created by milling or grinding in the upper part of the cylinder liner, as this will prevent the build-up of a new wear ridge.

Moreover, it has been noticed quite severe conditions when newly fitted piston rings are run in a cylinder liner with an existing wear edge, where the upper piston ring hammers against the wear edge at every piston stroke until the ring becomes chamfered at the top and collapses.

Chamfer on top ring
Chamfered top piston ring

MAN Diesel in cooperation with Chris Marine from Sweden, have developed a milling procedure for the reconditioning of the cylinder liners of large bore diesel engines by using a VKS type Chris Marine milling machine.

Chris Marine milling machine in working position

The job it will take around 1 hour or more, depending on the operator experience, and the most time consuming is the assembling and calibrating of the equipment as it must be in a perfect centered position. In approximately 15 minutes, the tool cuts a fine groove in the cylinder liner exactly in the desired position, but attention must be paid as the upper piston ring should not extend more than 2 mm above the lower groove edge.

Source: Chris Marine

However, not every vessel is equipped with such a tool and if such equipment is not available, an angle grinder with a coarse stone wheel of adequate thickness can be used. In this case, it should be noted that only a cylindrical grinding of 5 mm in height, just above the wear ridge, must be performed. The rest of the area, to the tp of the cylinder liner, will then act as a scrape down ring. The result will be good running conditions of the piston rings as the coke on the top land never touches the cylinder liner surface.

Efforts should be made to create the groove as specified before.

When hand grinding is applied with an angle grinder, it is recommended that an old piston ring is placed on top of the piston crown, and that the piston crown is turned to a position that enables the grind stone to rest on the piston ring while removing the wear ridge.

Grinding wear ridge

This method is highly recommended as although angle grinder is a very efficient tool, unfortunately it is impossible to control it vertically. This means that the cylinder liner running surface below the wear ridge can be easily damaged and often to the extent that the upper piston ring does not stand a chance of lasting the running-in period. If the piston ring collapse, gas blows by the ring, destroys the oil film and creates further blow by of the entire ring package and a scavenge fire could be the result.

MAN Diesel introduced the so-called Controlled Pressure Relief (CPR) top piston ring which highlighted the importance of removing wear ridges. The reason behind it is that the gas-tight seal of the CPR ring is susceptible to breakage in the event that a wear ridge is not removed when the piston is overhauled.

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